The present invention relates to a scanning electron microscope which allows observation of a sample by scanning the sample in synchronization with emission of electron rays. In particular, the present invention relates to a scanning electron microscope with which an observation object which is conventionally too large to fit in a sample chamber and therefore needs preliminary processing can be directly observed without any preliminary processing.
A scanning electron microscope (SEM) is conventionally known which enables the user to observe the composition or surface unevenness of a sample by converging electron rays emitted from an electron gun in a stepwise fashion with at least one electron lens to form a finely-focused flux of electrons, directing the formed convergent electron rays (electron beams) onto a sample as an observation object to scan the sample, and detecting secondary electrons and reflected electrons emitted from the sample in response to the scanning. One example of such a conventional scanning electron microscope is disclosed in JP-B-4349964.
FIG. 6 shows a configuration example of a conventionally known scanning electron microscope. The scanning electron microscope shown in FIG. 6 includes an electron gun 2 for emitting electron rays Z, scanning deflection means for converging the electron rays Z emitted from the electron gun 2 and deflecting the converged electron rays Z (electron beam) (in this example, a condenser lens 3, a scanning coil 5 and an objective lens 6 arranged in a multi-stage configuration correspond to the scanning deflection means), a secondary electron detector S1 for detecting secondary electrons emitted from a sample X as an observation object in response to the irradiation of the electron rays Z, the secondary electron detector S1 including a scintillator 8, a light guide F and a photomultiplier M, a reflected electron detector S2 for detecting reflected electrons emitted from the sample X as an observation object in response to the irradiation of the electron rays Z, a lens barrel 1 as a microscope main unit containing the electron gun 2, the scanning deflection means (3, 5, 6), the reflected electron detector S2 and the scintillator 8 with the photomultiplier M in an elongated shape protruded from a side thereof and connected to the scintillator 8 via the light guide F, a sample chamber 300 for containing the sample X as an observation object, a vacuum pump (not shown) for maintaining the interior of the lens barrel 1 and the sample chamber 300 in a vacuum state, and various control devices (not shown) for controlling respective parts of the scanning electron microscope.
The operation of the scanning electron microscope shown in FIG. 6 is described. First, the user opens a door K provided on a side of the sample chamber 300 and places a sample X as an observation object in the sample chamber 300. Then, the user closes the door K to close the sample chamber 300 in an airtight manner. The vacuum pump is activated to maintain the interior of the lens barrel 1 and the sample chamber 300 in a vacuum state. In other words, the air (atmosphere) in the sample chamber 300 and the lens barrel 1 is evacuated to create a vacuum in order to prevent the sample X in the sample chamber 300 from contacting the outside air prior to the emission of electron rays Z. In the lens barrel 1 maintained in a vacuum state as described above, the electron rays Z emitted from the electron gun 2 and accelerated as needed are converged and finely focused by at least one electron lens (electrostatic lens or electromagnetic lens using an electric field or magnetic field which can interfere with the electron rays, and the condenser lens 3, the objective lens 6 and so on correspond to the electron lens) and deflected by the scanning coil 5. Then, the electron rays Z are directed onto the sample X in the sample chamber 300 maintained in a vacuum state to scan a sample surface.
In response to the irradiation of the electron rays Z onto the sample X for scanning, secondary electrons and reflected electrons are emitted from the sample X. The secondary electrons and reflected electrons emitted from the sample X are detected by the detectors S1 and S2, respectively. A compositional image (COMPO image) or topographic image (TOPO image) is displayed on a display such as CRT (not shown) based on detection signals corresponding to the secondary electrons and reflected electrons detected by the detectors S1 and S2, and the user can observe the sample X by viewing the image displayed on the display. However, because the deflection control of the electron rays Z by the scanning coil 5 can provide only a limited observation area, the sample X is placed on a table O (which is also referred to as “stage”) movable vertically and horizontally, and tiltable and rotatable in the sample chamber 300 so that an observed part (observation surface) on the sample X can be significantly changed by controlling the drive of the table O.
In the automobile industry, a wide variety of materials and parts used in automobiles should be observed to identify a type of the paint applied to a vehicle body, analyze its components and investigate its secular changes or to investigate aging degradation of pulleys of a CVT (continuously variable transmission) that occurs during its continued use. For these purposes, a scanning electron microscope as described above is used. To use a conventional scanning electron microscope, however, as described above, a sample as an observation object should be placed in an airtight sample chamber because the interior of the sample chamber and the lens barrel must be maintained in a vacuum state during the emission of electron rays. Thus, an observation object which is too large to fit in the sample chamber (an automobile part such as a vehicle body or pulley) must be processed into a sample small enough to fit in the sample chamber by, for example, cutting the observation object. In other words, such an observation object cannot be observed unless the observation object is cut into a sample small enough to fit in the sample chamber.
However, once an observation object, such as an automobile part, is processed by, for example, cutting, the observation object unavoidably loses its function and cannot be used for its intended purpose any more. In other words, cutting a sample out of an observation object is destroying the observation object. For example, when a sample small enough to fit in the sample chamber is cut out of the bonnet of a vehicle as an observation object to observe the paint applied thereto, the bonnet has a hole and cannot be used for its intended purpose any more. Thus, the conventional scanning electron microscope is not suited for continued observation of changes over time of an observation object (automobile part as described above) which undergoes deterioration or wear as the vehicle is used. This is a problem stemming from the fact that the conventional scanning electron microscope allows observation of only a sample which is at least small enough to fit in the sample chamber and in which an observed part can be changed by driving the table.
One possible solution to the above problem is to eliminate the airtight sample chamber in the conventional scanning electron microscope (what is called an exposure electron microscope). In this case, the observation object does not have to be destroyed but the lens barrel as a main unit must be reduced in size and weight to construct an exposure electron microscope which allows observation of a large observation object, such as a vehicle body or pulley, by emission of electron rays without a sample chamber.
However, the conventional scanning electron microscope has a condenser lens or objective lens made up of a magnetic field coil in the lens barrel, and the condenser lens or objective lens cannot be reduced in size any more because of the structure of the magnetic field coil. It is, therefore, difficult to reduce the size and weight of the lens barrel itself. Another reason why reduction in weight of the lens barrel is difficult is that the lens barrel is required to have sufficient strength to support a photomultiplier having relatively large length and weight because the photomultiplier with an elongated shape is directly mounted on a side of the lens barrel. In addition, the necessity to create a vacuum in the lens barrel and on the sample surface which results from the elimination of the sample chamber and adverse effects during observation due to vibration of the lens barrel which is more likely to occur when the lens barrel is made light and small (for example, the compositional image or topographic image displayed on the display may be blurred and difficult to observe) may arise as new problems. Because of the above reasons, a light and small scanning electron microscope without a sample chamber which allows direct observation of an observation object too large to fit in a sample chamber, such as a vehicle body or pulley, has been neither provided nor suggested.